Plants alter surface charge and functional groups of their roots to adapt to acidic soil conditions

The rapid increase in soil acidification rate has led to a decrease in global agricultural productivity owing to the debilitating effects of Al and Mn toxicities. In this study, we investigated the adaptation of plants to acidic conditions by examining the behavior of plant roots grown in hydroponic solution and pot experiments at different pHs. The Mn(II) sorption by the roots was investigated and the mechanisms involved were deduced by analyzing the changes in the zeta potential and functional groups on the root surface. The exchangeable, complexed, and precipitated Mn(II) on plant roots were extracted sequentially with 1 M KNO₃, 0.05 M EDTA-2Na, and 0.01 M HCl. The results of hydroponic experiment indicated that plant roots subjected to NH₄⁺ treatment carried lower negative charge and fewer functional groups owing to acidic pH condition induced by NH₄⁺ uptake of roots, when compared with plant roots treated with NO₃^-. Similarly, in pot experiments, the surface negative charge and functional groups of plant roots cultured in soils with lower pH were fewer than those on plant roots cultured in soils with higher pH, with the former presenting less exchangeable and complexed Mn(II) sorption than the latter. Thus, alterations in the charge properties and number of functional groups on the surface of plant roots are some of the mechanisms used by plants to adapt to acidic soil condition.

Liming effects of poultry litter derived biochar on soil acidity amelioration and maize growth

Crop production in acid soils is facing enormous challenges due to low soil quality associated with an increase in the acidification rate and aluminum toxicity. Despite comprehensive prior work with biochar application on nutrient availability and crop productivity in acid soils, little information is available about the recommendation or standardization of biochar application rates that are more suitable for soil fertility improvement under different soil environments (physico-chemical properties) for maximizing the benefits of biochar applications and minimizing the potential environmental risk. Thus, the objective of this study was to investigate the effectiveness of poultry litter (PL) and poultry litter biochar (PLB) in ameliorating the fertility of acid soils through incubation and pot experiments. The soil was amended with different materials as follows; lime (1 g kg^-1), PL (5, 10 and 15 g kg^-1) and PLB (5, 10 and 15 g kg^-1) along with control (non-amended). A pot experiment was also conducted using similar treatments to observe the responses of maize crop to the different amendments. The results indicated an increase in the pH and a decrease in exchangeable acidity in lime, PL and PLB amended soils. Lower soil pH, base cations and soil available phosphorus (P), and higher exchangeable acidity were found in control than the amended soils. Compared to PL and lime, PLB achieved greater increase rate in soil pH and reduction rate in soil exchangeable acidity with increased soil exchangeable base cations. An increase in soil available calcium (Ca) was observed in the lime treatment, while in PL and PLB treatments, there was an increase in soil available Ca, magnesium (Mg), potassium (K) and P. Application of the amendments increased availability of nitrogen (N), P, K, Ca and Mg relative to the control for maize in the pot experiment. When PL and PLB amendments were compared, it was found that the PLB was the best choice for the amelioration of acid soils as well as nutrient uptake by maize plants. It is suggested that application of PLB at the rate of 15 g kg^-1 is suitable for maize growth in acid soils.

Biochars derived from crop straws increased the availability of applied phosphorus fertilizer for maize in Ultisol and Oxisol

The extensive use of phosphorus (P) fertilizer is a common practice due to the suboptimal P in variable charge soils for better crop growth. The aim of this study was to increase our understandings and disclose the mechanisms for increase in P availability in Ultisol and Oxisol by biochars derived from crop straws. Incubation and greenhouse pot experiments were conducted to attain the objective. Results from incubation study indicated that biochars derived from different crop straws increased P recovery in both Ultisol and Oxisol. Biochars increased the repulsion of soil surface to phosphate (PO₄^3-) anions due to increased soil CEC, and thus increased P recovery; acidic functional groups on biochars competed for soil sorption sites with PO₄^3-, and thus increased P recovery. While the formation of insoluble PO₄^3- with divalent cations of calcium (Ca) and magnesium (Mg) from the biochars reduced P recovery. P recovery was increased with increasing soil pH and biochar application rate for each biochar type. The lower content of Ca²⁺ and Mg²⁺ in rice straw biochar led to greater increase in P recovery than canola and peanut straw biochars. When two soils were compared with each other, P recovery was higher in Ultisol than Oxisol due to the presence of large amount of Fe and Al oxides in Oxisol. Results from pot experiment showed that plant dry matter yield and P recovery by maize were increased with increasing rate of rice straw biochar applied over control. Therefore, application of P fertilizer in rice straw biochar-amended soils will increase P availability to crops and thus crop yields in variable charge soils.

Higher cation exchange capacity determined lower critical soil pH and higher Al concentration for soybean

Low soil pH and aluminum (Al) toxicity induced by soil acidification are the main obstacles in many regions of the world for crop production. The purpose of this study was to reveal the mechanisms on how the properties of the soils derived from different parent materials play role on the determination of critical soil pH and Al concentration for soybean crops. A set of soybean pot experiment was executed in greenhouse with a soil pH gradient as treatment for each of four soils to fulfill the objectives of this study. The results indicated that plant growth parameters were affected adversely due to Al toxicity at low soil pH level in all soils. The critical soil pH varied with soil type and parent materials. They were 4.38, 4.63, 4.74, and 4.95 in the Alfisol derived from loss deposit, and the Ultisols derived from Quaternary red earth, granite, and Tertiary red sandstone, respectively. The critical soil exchangeable Al was 2.42, 1.82, 1.55, and 1.44 cmolc/kg for the corresponding soils. At 90% yield level, the critical Al saturation was 6.94, 10.36, 17.79, and 22.75% for the corresponding soils. The lower critical soil pH and Al saturation, and higher soil exchangeable Al were mainly due to greater soil CEC and exchangeable base cations. Therefore, we recommended that critical soil pH, soil exchangeable Al, and Al saturation should be considered during judicious liming approach for soybean production.

Mechanisms for Increasing the pH Buffering Capacity of an Acidic Ultisol by Crop Residue-Derived Biochars

The effects and underlying mechanisms of crop residue-derived biochars on the pH buffering capacity (pHbuff) of an acidic Ultisol, with low pHbuff, were investigated through indoor incubation and simulated acidification experiments. The incorporation of biochars significantly increased soil pHbuff with the magnitude of the increase dependent on acid buffering capacity of the biochar incorporated to the soil. Cation release, resulting from the protonation of carboxyl groups on biochar surfaces and the dissolution of carbonates, was the predominant mechanism responsible for the increase in soil pHbuff at pH 4.0-7.0 and accounted for >67% of the increased pHbuff. The reaction of protons with soluble silica (Si) in biochars derived from rice straw and corn stover also accounted for ∼20% of the pHbuff increase due to H₃SiO₄^- precipitation. In conclusion, the incorporation of crop residue-derived biochars into acidic soils increased soil pHbuff with peanut stover biochar being the most effective biochar tested.